This invention relates to magnetic fluid rotary seals designed to provide a pressure barrier in a variable pressure environment, and, more particularly, to magnetic fluid seals designed to prevent bursting during operation.
Magnetic fluid rotary seals have been used for many years in different environments. The prototypical seal device of this type is described in U.S. Pat. No. 3,620,584. A comprehensive review describing numerous applications and modifications of the seal is presented in Chapter 5 authored by Kuldip Raj in Magnetic Fluids and Applications Handbook, B. Berkovski, Ed., Begell House, Inc., New York (1996). A treatment describing the synthesis, make-up, fluid and magnetic properties, and flow of magnetic fluids is given in the monograph Ferrohydrodynamics, R. E. Rosensweig, Cambridge University Press, New York (1985), reprinted by Dover Publications, Inc., Mineola, N.Y. (1997).
As stated in the aforementioned publications, and as used herein, a magnetic fluid is an ultra-stable colloidal dispersion of approximately 10 nm size magnetic particles in a liquid carrier. Such colloidal magnetic fluids are also known as xe2x80x9cferrofluids.xe2x80x9d The particles are sufficiently small that they are prevented from settling in gravitational or magnetic fields by thermal motion. A surface coating of adsorbed surfactant(s) or electric charges prevents agglomeration of particles to one another so that the colloids are stable over a long period of time.
Such a fluid may be used to create a gas tight seal. In prior art seal devices, magnetic fluid is retained as a ring in a gap, for example, a gap surrounding a cylindrical rotating shaft, by a magnetic field in the gap. The magnetic fluid serves as a barrier to the passage of gas along the shaft while permitting rapid rotation of the shaft, if desired. For a given magnetic field, the amount of pressure differential across the fluid ring that the seal can support is primarily determined by the magnetic field intensity in the gap.
FIG. 1 illustrates components and magnetic field lines of such a conventional magnetic fluid seal having permanent magnet 1 with annular, permeable pole pieces 2 and 8 defining gaps, such as gaps 3 and 7, between pole pieces 2 and 8 and permeable rotating shaft 4. Magnetic lines of flux 5 circulate through the magnetic circuit formed by magnet 1, pole piece 2, gap 3, shaft 4, gap 7 and pole piece 8. The magnetic flux lines 5 concentrate to a high intensity in the gaps defined by a tooth, such as tooth 6. Magnetic fluid is magnetically retained in discrete rings such as rings 9, 11 and 13 circling the shaft 4 and bridging the gaps 3 and 7 between teeth and a pole piece.
The rings 9, 11 and 13 of magnetic fluid prevent the flow of gas under pressure from a region 14 to a lower pressure region 15. A ring of magnetic fluid is referred to as a seal xe2x80x9cstage.xe2x80x9d The seal stages are separated by gas filled interstage regions, such as regions 10 and 12. In variations of this conventional seal, the teeth may be recessed in the shaft rather than extending from the surface, teeth may be located on a pole block opposite the smooth surface of a shaft, teeth may be present at the gaps of both pole blocks, teeth may be tapered or otherwise shaped, and multiple magnets may be employed. For simplicity, FIG. 1 omits bearings supporting the shaft, housing, static seals, retaining rings, etc. that are part of a total seal package, as these elements are well known to one skilled in the art.
In magnetic fluid seals, the magnetic field intensity in the gaps is, in turn, determined by the configuration of the magnetic circuit that generates the field. The intensity of magnetic field established in the gap depends on the magnetomotive force of the magnet and the magnetic reluctance of the magnetic circuit elements and is analogous to the flow of current in a resistive electrical circuit containing a source of electromotive force, as is well known in the design of magnetic and electrical systems. In conventional seals, the magnetic circuit insures that the seals operate with a constant magnetic field in the seal gaps.
A single fluid ring, which constitutes one stage, can withstand only a limited pressure differential, and when this differential is exceeded, the ring xe2x80x9cbursts.xe2x80x9d When a burst occurs, a leakage path develops through the fluid ring and allows gas to pass by the seal. This process is illustrated in FIGS. 2A-C. As shown in FIG. 2A, with no pressure differential across the stage, the cross-section of a ring of magnetic fluid is symmetrically positioned, bridging the gap between a pole piece and the shaft. When a pressure differential is applied across the seal stage, the ring of fluid is displaced toward the low-pressure side as shown in FIG. 2B. Due to fringing, magnetic field intensity is weakest at the shaft surface. When the pressure difference is excessive, a leakage path opens up adjacent to the shaft surface with the magnetic fluid lifted away from the surface. The bursting condition is illustrated in FIG. 2C.
Accordingly, a seal designed to support a high-pressure differential is typically equipped with multiple stages of rings arranged in series longitudinally on the shaft. Upon initial exposure to the pressure differential, the outer seal rings are exposed to large pressure differences and a seal ring may burst temporarily and distribute excess pressure to the next stage. When pressure-holding capacity of that next stage is exceeded, the ring associated with that stage bursts and permits transfer of gas to the subsequent stage. This process continues until the seal stages reach equilibrium.
After a seal stage bursts, the pressure difference across the seal stage is reduced and the integrity of the fluid ring in that stage is restored as the fluid in the gap reseals itself. Thus, after the initial application of pressure across the seal, the seal stages will reach equilibrium and reseal them selves. However, each seal stage that has burst will subsequently operate near its burst condition. Later, if a pressure fluctuation occurs that increases the pressure difference across a stage, or if a condition develops that decreases the pressure holding capacity of a stage (for mechanical, thermal, magnetic or other reasons), the seal stage may burst during operation. This second burst may then release a volume of gas trapped in the interstage region into a process chamber, e.g. a vacuum chamber in which integrated circuits are fabricated and the gas can be detrimental to the processing operation.
FIGS. 3A-3D illustrate the interstage pressurization conditions in a conventional magnetic fluid seal used in a material processing system. The figures show the seal during sequential processing steps in an environment where the pressure varies between vacuum and atmospheric pressure. FIG. 3A depicts the seal prior to establishing vacuum at one end. The interstage regions all hold air at one atmosphere pressure. The distribution of interstage pressurization after pumpdown is illustrated in FIG. 3B. Typically, about 3 psi of pressure differential is established across each seal stage that previously has burst and resealed as discussed in relation to FIG. 2. A number of stages remain in the unburst condition with no pressure difference across them. They furnish a reserve or margin of safety and ensure long life of the seal should magnetic fluid evaporate or otherwise be removed from the working stages. During the course of processing, the vacuum vessel may be backfilled, or re-pressurized, in order to remove processed material. FIG. 3C illustrates the distribution of interstage pressures following a backfill. Following a subsequent pumpdown, the interstage pressurizations return to their previous pumped down values as shown in FIG. 3D. It will be understood that the pressure changes presented are notional and intended for illustration only, and may not coincide with conditions in an actual seal device.
Prior-art, conventional magnetic fluid seals employed in high-vacuum systems, typically those of 10xe2x88x926 Torr or higher vacuum, either under static or dynamic conditions, may permit a burst of air through the seal and its introduction into the vacuum system. The burst of air occurs intermittently, depending on the seal design, the amount of magnetic fluid in the seal, and operating conditions. For example, when the magnetic fluid seal is started or is employed for the first time after being in a static condition, a burst of air is usually introduced into the vacuum system. In modern, high-vacuum processing systems, these air bursts present limitations on the employment of magnetic fluid multiple-stage seals. A detailed discussion of these aspects may be found in U.S. Pat. No. 4,407,518.
Several prior art attempts have been made to solve the bursting problem. For example U.S. Pat. No. 4,407,518 discloses a non-bursting multiple-stage seal and system in which pumping with pressure monitoring is employed to maintain the differential pressure across the end stage nearest the vacuum chamber at a predetermined level sufficient to prevent the stage from bursting. Often a separate vacuum pump is required in addition to a pressure sensor, feedback control, and conduits. Accordingly, the system is relatively expensive and subject to maintenance and down time.
U.S. Pat. No. 4,445,696 describes a magnetic fluid seal having the goal of non-bursting operation. All the seal stages are located on the atmospheric side of the magnet, and on the vacuum side of the magnet, the space between shaft and pole block is made very small so that flow through it, if any occurs, takes place slowly. Thus, the patent provides for a reduced flow rate of leakage gas into the vacuum vessel in the event of a burst. While this seal minimizes the impact of a burst, it does not prevent bursts.
U.S. Pat. No. 4,605,233 discloses a magnetic fluid rotary seal employing a multiplicity of permanent magnets disposed in magnetic opposition to each other that drive flux through pole pieces located in-between. The stage nearest the vacuum side is configured to withstand a larger pressure differential than the other stages of the seal. However, the design is no better suited for vacuum sealing than a conventional magnetic fluid seal as the stage nearest the vacuum end must burst and reseal to establish initial operation, and hence operates near its burst condition.
U.S. Pat. No. 5,340,122 describes another form of differentially-pumped magnetic fluid seal in which the bearings supporting the shaft are isolated from exposure to the vacuum chamber thus preventing contamination of the chamber due to outgasing of bearing materials. A multistage seal withstands rough-vacuum/atmosphere pressure differential while a separate,.one stage seal containing its own magnet source is operated with pressure differential of high-vacuum/rough vacuum. A space between the two seal regions is evacuated with a roughing pump to ensure the presence of a small pressure difference across the single stage seal that therefore is prevented from bursting. Again, a complex and expensive system is needed to achieve the benefits the device confers.
In view of the foregoing, it is desirable to achieve non-bursting performance of a magnetic fluid seal in an inexpensive, compact device.
In accordance with the principles of the invention, the magnetic field intensity in one or more of the seal gaps is varied in time. During the initial application of pressure across the seal, the magnetic field intensity is reduced, allowing the seal stages to burst and reach equilibrium. Thereafter, the magnetic field intensity is increased, in turn increasing the pressure capacity of each seal stage so that each seal stage is rendered non-bursting.
The magnetic field intensity in the seal gaps is controlled by changing the magnetic reluctance of the magnetic circuit that creates the field in the gaps. In one embodiment, a space is introduced between the magnet and one of the pole pieces in order to increase the magnetic reluctance. The lower intensity field in the seal gaps is established when the space is introduced and the higher intensity field is established when the space is removed.
Another embodiment changes the reluctance by changing the magnetic reluctance of one of the magnetic circuit elements. For example, the rotating shaft may be hollowed out to increase its magnetic reluctance. The lower intensity of magnetic field in the seal gaps results from a relatively large reluctance presented by the thin walls of the hollow shaft. In order to increase the field strength in the seal gaps, a flux intensifier comprising a soft ferromagnetic is inserted into the shaft. The flux intensifier reduces the magnetic circuit reluctance.
A further embodiment uses a flux diverter, for example, made of soft ferromagnetic material, placed across the magnet. A portion of the magnetic induction passing through the magnet runs through the diverter causing the magnet to operate at a lower coercivity that, in turn, reduces the intensity of magnetic field in the seal gaps. When the diverter is removed, the magnetic field intensity in the seal gaps increases. This embodiment is particularly useful with seals employing radially magnetized permanent magnets.
Still another embodiment of this invention uses a wound coil of electrical conductor configured to reduce the magnetomotive driving force in the magnetic circuit when current flows in the coil. The relatively low magnetic field intensity in the gaps is established when current flows and the higher magnetic intensity is established by switching the current off. This embodiment has the advantage that current need only be supplied to the coil during the short period when the seal is put into service. Subsequently, the seal may be operated in the nonbursting condition for any length of time.